JP2014049372A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery high in safety, further high in capacity, and also excellent in cycle characteristics.SOLUTION: In a lithium ion secondary battery, a lithium-containing complex oxide containing lithium and nickel is included as a positive electrode active material, and a positive electrode active material 1 having a mole fraction a1 of nickel in the lithium-containing complex oxide of 0.30-0.59, a positive electrode active material 2 having a mole fraction a2 of nickel in the lithium-containing complex oxide of 0.60-0.79. and a positive electrode active material 3 having a mole fraction a3 of nickel in the lithium-containing complex oxide of 0.80-0.95 are included. The total sum Nt of molar content ratios of nickel satisfies a relation: 0.6<Nt<0.75.

Description

  The present invention relates to a lithium ion secondary battery having high safety, high capacity, and excellent cycle characteristics.

  As a high-capacity electrode active material in a lithium ion secondary battery, a lithium-containing composite oxide containing nickel is known as a positive electrode active material. It is also known that a plurality of positive electrode active materials having different composition ratios and particle sizes of nickel in a lithium-containing composite oxide are mixed to improve capacity characteristics, output characteristics, and safety, respectively (Patent Documents 1 and 2). ).

JP 2010-86693 A JP 2011-146132 A

  However, in the above-mentioned patent document, the evaluation of the positive electrode alone in the cell using metallic lithium as the counter electrode is limited, and various characteristics such as capacity, output, and cycle of the mounted battery such as a cylindrical shape, and safety such as overcharge and external short circuit. There was still room for improvement in terms of securing.

  As a result of studying for the purpose of providing a lithium ion secondary battery having high safety and high capacity, the above object is achieved by using three types of lithium-containing composite oxides having different nickel molar ratios in combination. Further, the present invention has been found to be effective in improving cycle characteristics at a high current.

A lithium ion secondary battery of the present invention is a non-aqueous electrolyte secondary battery in which a bottomed can, an electrode body configured by winding a negative electrode, a positive electrode, and a separator, and a non-aqueous electrolyte at least,
The negative electrode includes a material containing Si and O as constituent elements (however, the atomic ratio x of O to Si is 0.5 ≦ x ≦ 1.5) as a negative electrode active material, A positive electrode active material 1 containing a lithium-containing composite oxide containing lithium and nickel as a positive electrode active material, wherein the molar ratio a1 of nickel in the lithium-containing composite oxide is 0.30 to 0.59; The positive electrode active material 2 in which the molar ratio a2 of nickel in the composite oxide is 0.60 to 0.79, and the molar ratio a3 of nickel in the lithium-containing composite oxide is 0.80 to 0.95. The total Nt of the nickel mole content ratios represented by Formula 1 including at least a certain positive electrode active material 3 satisfies 0.6 <Nt <0.75.

(Formula 1)

ai: Nickel molar ratio contained in each positive electrode active material of positive electrode active materials 1 to 3 mi: Content mass ratio of each positive electrode active material of positive electrode active materials 1 to 3 contained in all positive electrode active materials

  According to the present invention, it is possible to provide a lithium ion secondary battery that has high capacity and high safety and is excellent in charge / discharge cycle characteristics at a high current.

FIG. 1 is a longitudinal sectional view according to an embodiment of a lithium ion secondary battery of the present invention.

  Hereinafter, embodiments of the present invention will be described in detail. However, these are merely examples of embodiments of the invention, and the present invention is not limited to these contents.

<Positive electrode>
As the positive electrode according to the lithium ion secondary battery of the present invention, for example, one having a structure having a positive electrode mixture layer containing a positive electrode active material, a binder, a conductive auxiliary agent and the like on one side or both sides of a current collector can be used.

<Positive electrode active material>
The positive electrode active material according to the lithium ion secondary battery of the present invention is a lithium-containing composite oxide containing lithium and a transition metal, and is a lithium-containing composite oxide containing nickel as the transition metal. Here, the lithium-containing composite oxide containing nickel is represented by the following general composition formula (1).
_{Li x Ni ai Co bi M ci} O 2 (1)
Here, in the formula, x: 0.3 to 1.2, and ai: the nickel molar ratio of the i component among the positive electrode active materials 1 to 3, respectively. Furthermore, the molar ratio a1 of nickel of the positive electrode active material 1: 0.30 to 0.59, the molar ratio of nickel of the positive electrode active material 2 a2: 0.60 to 0.79, the molar ratio of nickel of the positive electrode active material 3 a3: 0.80 to 0.95.

  In the formula, M represents a metal other than nickel (Ni) and cobalt (Co). For example, manganese (Mn), titanium (Ti), chromium (Cr), iron (Fe), copper (Cu), silver (Ag), thallium (Ta), niobium (Nb), zirconium (Zr), and other transition metals, and other than transition metals, boron (B), phosphorus (P), zinc (Zn), aluminum (Al) Calcium (Ca), strontium (Sr), barium (Ba) germanium (Ge), tin (Sn), magnesium (Mg), and the like. These metals may be a single species or may contain two or more species.

  The positive electrode active material 1 having a low nickel mole fraction is excellent in safety such as overcharge resistance and external short-circuit resistance, but has a slightly lower capacity. On the other hand, the positive electrode active material 3 having a high nickel mole fraction is superior in capacity, but has low safety at high temperatures, and thus has a difficulty in safety. Thus, it has been found that the positive electrode active materials 2 having a nickel molar ratio corresponding to the middle between the positive electrode active materials 1 and 3 can be used together to bring out the respective characteristics of the positive electrode active materials 1 and 3.

  Furthermore, the positive electrode active material according to the lithium ion secondary battery of the present invention is characterized in that the sum Nt of nickel mole content ratios represented by Formula 1 satisfies 0.6 <Nt <0.75.

(Formula 1)

ai: Nickel molar ratio included in each positive electrode active material of positive electrode active materials 1-3 mi: Content ratio of each positive electrode active material in positive electrode active materials 1-3 included in all positive electrode active materials Nt is 0.6 As a result, the capacity of the lithium ion secondary battery, which is the first object of the present invention, can be increased. Moreover, the high safety | security of the lithium ion secondary battery which is the 2nd objective of this invention can be achieved by making Nt less than 0.75. While setting Nt in the above range and using each of the positive electrode active materials 1 to 3 in detail, the charge / discharge cycle characteristics at a high current, which is the third object of the present invention, are also unclear. I found that it can be improved.

The average particle size of the lithium-containing composite oxide (positive electrode active materials 1 to 3) containing nickel as a transition metal used in the present invention is preferably 4 to 15 μm, particularly preferably 6 to 10 μm. By setting the average particle size to 4 μm or more, it is possible to increase the density of the positive electrode and to suppress excessive reaction with the electrolytic solution and the like. Further, by setting the thickness to 15 μm or less, charge / discharge characteristics and cycle characteristics at a high current can be improved. These particles may be secondary aggregates in which primary particles are aggregated, and the average particle diameter in this case means the average particle diameter of the secondary aggregates. The average particle size of various particles is, for example, an average particle size measured by dispersing these fine particles in a medium that does not dissolve the resin using a laser scattering particle size distribution meter (for example, “LA-920” manufactured by Horiba, Ltd.). D _50% .

  Furthermore, Di is a ratio of the average particle diameter Dave (μm) of all the positive electrode active materials, that is, the positive electrode active materials 1 to 3 and the average particle diameter Di of each of the positive electrode active materials 1 to 3. / Dave is preferably 0.7 to 1.3. It has been found that the object of the present invention can be achieved more suitably by making the average particle size of each positive electrode active material equal within a range satisfying the above conditions. Details are unknown, but by making the average particle size of each positive electrode active material equal within the range that satisfies the above conditions, the migration path of lithium ions from the bulk surface of the positive electrode active material to the inside can be made uniform. As a result, it is considered that the uniformity of the reactivity is ensured.

The specific surface area by the BET method of all the positive electrode active materials is 0.1 to 0.4 m ² / g for reasons such as ensuring reactivity with lithium ions and suppressing side reactions with the electrolyte. It is preferable to do. The specific surface area by the BET method can be measured by using a specific surface area measuring device by the nitrogen adsorption method (“Macsorb HM model-1201” manufactured by Mounttech).

<Conductive aid for positive electrode mixture layer>
The conductive auxiliary for the positive electrode mixture layer relating to the positive electrode of the lithium ion secondary battery of the present invention may be any one that is chemically stable in the lithium ion secondary battery. For example, graphite such as natural graphite and artificial graphite, acetylene black, ketjen black (trade name), carbon black such as channel black, furnace black, lamp black and thermal black; conductive fiber such as carbon fiber and metal fiber; aluminum Metallic powders such as powders; Fluorinated carbon; Zinc oxide; Conductive whiskers made of potassium titanate; Conductive metal oxides such as titanium oxide; Organic conductive materials such as polyphenylene derivatives; One species may be used alone, or two or more species may be used in combination. Among these, highly conductive graphite and carbon black excellent in liquid absorption are preferable. Further, the form of the conductive auxiliary agent is not limited to primary particles, and secondary aggregates and aggregated forms such as chain structures can also be used. Such an assembly is easier to handle and has better productivity.

<Binder for positive electrode mixture layer>
As the binder of the positive electrode mixture layer relating to the positive electrode of the lithium ion secondary battery of the present invention, any one of a thermoplastic resin and a thermosetting resin can be used as long as it is chemically stable in the lithium ion secondary battery. Can also be used. For example, polyvinylidene fluoride (PVDF), tetrafluoroethylene-vinylidene fluoride copolymer (P (TFE-VDF)), polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyhexafluoropropylene (PHFP), styrene butadiene Rubber, tetrafluoroethylene-hexafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), propylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer (ECTFE), It is ethylene - acrylic acid copolymer, ethylene - methacrylic acid copolymer, ethylene - methyl acrylate copolymer, ethylene - methyl methacrylate copolymer and the like Na ion crosslinked product thereof copolymer.

<Positive electrode mixture layer, current collector, etc.>
The positive electrode is prepared, for example, by preparing a paste-like or slurry-like positive electrode mixture-containing composition in which the positive electrode active material, binder and conductive additive described above are dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP). (However, the binder may be dissolved in a solvent), which is applied to one or both sides of the current collector, dried, and then subjected to a calendering process as necessary. However, the manufacturing method of the positive electrode is not limited to the above method, and may be manufactured by other manufacturing methods.

  As the composition of the positive electrode mixture layer, for example, the amount of the positive electrode active material is preferably 60 to 95% by mass, the amount of the binder is preferably 1 to 15% by mass, and the amount of the conductive auxiliary agent is 3 It is preferable that it is -20 mass%.

Moreover, it is preferable that the thickness of an electrode mixture layer is 15-200 micrometers per single side | surface of a collector after a calendar process. Furthermore, after the calendar treatment, the density of the electrode mixture layer is preferably 3.2 g / cm ³ or more, and more preferably 3.6 g / cm ³ or more. By using an electrode having such a high-density electrode mixture layer, higher capacity can be achieved. However, if the density of the electrode mixture layer is too large, the porosity becomes small and the permeability of the non-aqueous electrolyte may decrease, so the density of the electrode mixture layer after the press treatment is 4. It is preferably 2 g / cm ³ or less. In addition, as a calendar process, it can roll-press with the linear pressure of about 1-30 kN / cm, for example, It can be set as the electrode mixture layer which has the said density by such a process.

  Moreover, the density of the electrode mixture layer as used in the present specification is a value measured by the following method. The electrode is cut into a predetermined area, the mass is measured using an electronic balance having a minimum scale of 0.1 mg, and the mass of the electrode mixture layer is calculated by subtracting the mass of the current collector. On the other hand, the total thickness of the electrode was measured at 10 points with a micrometer having a minimum scale of 1 μm, and the volume of the electrode mixture layer was calculated from the average value obtained by subtracting the thickness of the current collector from these measured values and the area. To do. Then, the density of the electrode mixture layer is calculated by dividing the mass of the electrode mixture layer by the volume.

  The current collector can be the same as that used for the positive electrode of a conventionally known lithium ion secondary battery. For example, an aluminum foil having a thickness of 10 to 30 μm is preferable.

<Negative electrode>
For the negative electrode according to the lithium ion secondary battery of the present invention, for example, a negative electrode mixture layer containing a negative electrode active material, a binder or the like is used on one or both sides of the current collector. The negative electrode active material related to the negative electrode is not particularly limited as long as it is a negative electrode used in a conventionally known lithium ion secondary battery, that is, a negative electrode containing an active material capable of occluding and releasing lithium ions. For example, as an active material, graphite, pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, mesocarbon microbeads (MCMB), carbon fibers, etc. having an average particle size of about 5 to 30 μm One or a mixture of two or more carbon-based materials capable of occluding and releasing lithium ions is used. In addition, elements such as Si, Sn, Ge, Bi, Sb and In and alloys thereof, lithium-containing nitrides, oxides and other compounds that can be charged and discharged at a low voltage close to lithium metal, or lithium metals and lithium / aluminum alloys Can also be used as a negative electrode active material. Among these, since the composite of SiO _x and carbon material has a high capacity, if the same high capacity lithium-containing composite oxide represented by, for example, the general composition formula (2) is combined as the positive electrode active material, the high capacity Battery can be provided. Further, it is more preferable to use a negative electrode active material in which a composite of SiO _x and carbon material is combined with a graphitic carbon material having excellent load characteristics and cycle characteristics.

The SiO _x may contain Si microcrystal or amorphous phase. In this case, the atomic ratio of Si and O is a ratio including Si microcrystal or amorphous phase Si. That is, SiO _x includes a structure in which Si (for example, microcrystalline Si) is dispersed in an amorphous SiO ₂ matrix, and this amorphous SiO ₂ is dispersed in the SiO ₂ matrix. It is sufficient that the atomic ratio x satisfies 0.5 ≦ x ≦ 1.5 in combination with Si. For example, in the case of a material in which Si is dispersed in an amorphous SiO ₂ matrix and the material has a molar ratio of SiO ₂ to Si of 1: 1, x = 1, so that the structural formula is represented by SiO. The In the case of a material having such a structure, for example, in X-ray diffraction analysis, a peak due to the presence of Si (microcrystalline Si) may not be observed, but when observed with a transmission electron microscope, the presence of fine Si Can be confirmed.

Then, SiO _x is a complex complexed with carbon materials, for example, it is desirable that the surface of the SiO _x is coated with a carbon material. As described above, since SiO _x has poor conductivity, when using it as a negative electrode active material, from the viewpoint of securing good battery characteristics, a conductive material (conductive aid) is used, and SiO _x in the negative electrode is used. It is necessary to form an excellent conductive network by mixing and dispersing the material and the conductive material well. If complexes complexed with carbon material SiO _x, for example, simply than with a material obtained by mixing a conductive material such as SiO _x and the carbon material, good conductive network in the negative electrode Formed.

The complex of the SiO _x and the carbon material, as described above, other although the surface of the SiO _x coated with carbon material, such as granules of SiO _x and the carbon material can be cited.

In addition, since the composite in which the surface of SiO _x is coated with a carbon material is further combined with a conductive material (carbon material or the like), a better conductive network can be formed in the negative electrode. Thus, it is possible to realize a non-aqueous secondary battery with higher capacity and better battery characteristics (for example, charge / discharge cycle characteristics). The complex of the SiO _x and the carbon material coated with a carbon material, for example, like granules the mixture was further granulated with SiO _x and the carbon material coated with a carbon material.

Further, as SiO _x whose surface is coated with a carbon material, the surface of a composite (for example, a granulated body) of SiO _x and a carbon material having a smaller specific resistance value is further coated with a carbon material. Those can also be preferably used. In a state where SiO _x and the carbon material are dispersed in the granulated body, a better conductive network can be formed. Therefore, in a lithium ion secondary battery having a negative electrode containing SiO _x as a negative electrode active material, Battery characteristics such as load discharge characteristics can be further improved.

Preferred examples of the carbon material that can be used to form a composite with SiO _x include carbon materials such as low crystalline carbon, carbon nanotubes, and vapor grown carbon fibers.

The details of the carbon material include at least one selected from the group consisting of fibrous or coiled carbon materials, carbon black (including acetylene black and ketjen black), artificial graphite, graphitizable carbon, and non-graphitizable carbon. A seed material is preferred. Fibrous or coil-like carbon materials are preferable in that they easily form a conductive network and have a large surface area. Carbon black (including acetylene black and ketjen black), graphitizable carbon, and non-graphitizable carbon have high electrical conductivity and high liquid retention, and even if SiO _x particles expand and contract. This is preferable in that it has a property of easily maintaining contact with the particles.

A graphite carbon material used in combination with SiO _x as a negative electrode active material can also be used as a carbon material related to a composite of SiO _x and a carbon material. Graphite carbon material, like carbon black, has high electrical conductivity and high liquid retention, and even when SiO _x particles expand and contract, they easily maintain contact with the particles. Therefore, it can be preferably used for forming a complex with SiO _x .

Among the carbon materials exemplified above, a fibrous carbon material is particularly preferable for use when the composite with SiO _x is a granulated body. Fibrous carbon material can follow the expansion and contraction of SiO _x with the charging and discharging of the battery due to the high shape is thin threadlike flexibility, also because bulk density is large, many and SiO _x particles It is because it can have a junction. Examples of the fibrous carbon include polyacrylonitrile (PAN) -based carbon fiber, pitch-based carbon fiber, vapor-grown carbon fiber, and carbon nanotube, and any of these may be used.

The fibrous carbon material can also be formed on the surface of the SiO _x particles by, for example, a vapor phase method.

The specific resistance value of SiO _x is usually 10 ³ to 10 ⁷ kΩcm, while the specific resistance value of the carbon material exemplified above is usually 10 ^{−5 to} 10 kΩcm. The composite of SiO _x and the carbon material may further have a material layer (a material layer containing non-graphitizable carbon) that covers the carbon material coating layer on the particle surface.

When a composite of SiO _x and a carbon material is used for the negative electrode, the ratio of SiO _x and the carbon material is based on SiO _x : 100 parts by mass from the viewpoint of satisfactorily exerting the effect of the composite with the carbon material. The carbon material is preferably 5 parts by mass or more, and more preferably 10 parts by mass or more. Further, in the composite, if the ratio of the carbon material to be combined with SiO _x is too large, it may lead to a decrease in the amount of SiO _x in the negative electrode mixture layer, and the effect of increasing the capacity may be reduced. SiO _x: relative to 100 parts by weight, the carbon material, and more preferably preferably not more than 50 parts by weight, more than 40 parts by weight.

The composite of the SiO _x and the carbon material can be obtained, for example, by the following method.

First, a manufacturing method in the case of combining SiO _x will be described. A dispersion liquid in which SiO _x is dispersed in a dispersion medium is prepared, and sprayed and dried to produce composite particles including a plurality of particles. For example, ethanol or the like can be used as the dispersion medium. It is appropriate to spray the dispersion in an atmosphere of 50 to 300 ° C. In addition to the above method, similar composite particles can be produced also by a granulation method by a mechanical method using a vibration type or planetary type ball mill or rod mill.

Incidentally, the SiO _x, in the case of manufacturing a granulated body with small carbon material resistivity value than SiO _x is adding the carbon material in the dispersion liquid of SiO _x are dispersed in a dispersion medium, the dispersion by using a liquid, by a similar method to the case of composite of SiO _x may be a composite particle (granule). Further, by granulation process according to the similar mechanical method, it is possible to produce a granular material of the SiO _x and the carbon material.

Next, when the surface of SiO _x particles (SiO _x composite particles or a granulated body of SiO _x and a carbon material) is coated with a carbon material to form a composite, for example, the SiO _x particles and the hydrocarbon-based material The gas is heated in the gas phase, and carbon generated by pyrolysis of the hydrocarbon-based gas is deposited on the surface of the particles. As described above, according to the vapor deposition (CVD) method, the hydrocarbon-based gas spreads to every corner of the composite particle, and the surface of the particle and the pores in the surface are thin and contain a conductive carbon material. Since a uniform film (carbon material coating layer) can be formed, the SiO _x particles can be imparted with good conductivity with a small amount of carbon material.

In the production of SiO _x coated with a carbon material, the processing temperature (atmosphere temperature) of the vapor deposition (CVD) method varies depending on the type of hydrocarbon gas, but usually 600 to 1200 ° C. is appropriate. Among these, the temperature is preferably 700 ° C. or higher, and more preferably 800 ° C. or higher. This is because the higher the treatment temperature, the less the remaining impurities, and the formation of a coating layer containing carbon having high conductivity.

  As the liquid source of the hydrocarbon-based gas, toluene, benzene, xylene, mesitylene and the like can be used, but toluene that is easy to handle is particularly preferable. A hydrocarbon-based gas can be obtained by vaporizing them (for example, bubbling with nitrogen gas). Moreover, methane gas, acetylene gas, etc. can also be used.

In addition, after the surface of SiO _x particles (SiO _x composite particles or a granulated body of SiO _x and a carbon material) is covered with a carbon material by a vapor deposition (CVD) method, a petroleum-based pitch or a coal-based pitch is used. At least one organic compound selected from the group consisting of a thermosetting resin and a condensate of naphthalene sulfonate and aldehydes is attached to a coating layer containing a carbon material, and then the organic compound is attached. The obtained particles may be fired.

Specifically, a dispersion liquid in which a SiO _x particle (SiO _x composite particle or a granulated body of SiO _x and a carbon material) coated with a carbon material and the organic compound are dispersed in a dispersion medium is prepared, The dispersion is sprayed and dried to form particles coated with the organic compound, and the particles coated with the organic compound are fired.

  An isotropic pitch can be used as the pitch, and a phenol resin, a furan resin, a furfural resin, or the like can be used as the thermosetting resin. As the condensate of naphthalene sulfonate and aldehydes, naphthalene sulfonic acid formaldehyde condensate can be used.

As a dispersion medium for dispersing the SiO _x particles coated with the carbon material and the organic compound, for example, water or alcohols (ethanol or the like) can be used. It is appropriate to spray the dispersion in an atmosphere of 50 to 300 ° C. The firing temperature is usually 600 to 1200 ° C., preferably 700 ° C. or higher, and more preferably 800 ° C. or higher. This is because the higher the processing temperature, the less the remaining impurities, and the formation of a coating layer containing a high-quality carbon material with high conductivity. However, the processing temperature needs to be lower than the melting point of SiO _x .

Examples of the graphitic carbon material used as the negative electrode active material together with the composite of SiO _x and the carbon material include natural graphite such as flake graphite; graphitizable carbon such as pyrolytic carbons, MCMB, and carbon fiber. And artificial graphite obtained by graphitizing at 2800 ° C. or higher.

In the negative electrode according to the present invention, from the viewpoint of satisfactorily ensuring the effect of the high capacity by using a SiO _x, the content of the complex of the SiO _x and the carbon material in the anode active material, 0 It is preferably 0.01% by mass or more, more preferably 1% by mass or more, and more preferably 3% by mass or more. Further, from the viewpoint of better avoiding the problem due to the volume change of SiO _x accompanying charge / discharge, the content of the composite of SiO _x and carbon material in the negative electrode active material is preferably 20% by mass or less. More preferably, it is 15 mass% or less.

The negative electrode according to the present invention is sufficiently kneaded by adding an appropriate solvent (dispersion medium) to a mixture (negative electrode mixture) containing a composite of SiO _x and a carbon material, a graphitic carbon material, and a binder. The obtained paste-like or slurry-like negative electrode mixture-containing composition is applied to one side or both sides of the current collector, and the solvent (dispersion medium) is removed by drying or the like, thereby having a predetermined thickness and density. It can be obtained by forming a layer. In addition, the negative electrode which concerns on this invention is not restricted to what was obtained by the said manufacturing method, The thing manufactured by the other manufacturing method may be used.

<Binder of negative electrode mixture layer>
Examples of the binder used in the negative electrode mixture layer include starch, polyvinyl alcohol, polyacrylic acid, carboxymethylcellulose (CMC), hydroxypropylcellulose, regenerated cellulose, diacetylcellulose, and other polysaccharides and modified products thereof; polyvinylchloride, Thermoplastic resins such as polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyamide and their modified products; polyimide; ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene Rubber (SBR), butadiene rubber, polybutadiene, fluororubber, polyethylene oxide and other polymers having rubbery elasticity, and modified products thereof. May be used alone or two or more al.

<Conductive aid for negative electrode mixture layer>
A conductive material may be further added to the negative electrode mixture layer as a conductive aid. Such a conductive material is not particularly limited as long as it does not cause a chemical change in the lithium ion secondary battery. For example, carbon black (thermal black, furnace black, channel black, ketjen black, acetylene black) Etc.), carbon fibers, metal powder (copper, nickel, aluminum, silver, etc.), metal fibers, polyphenylene derivatives (described in JP-A-59-20971), and the like are used alone or in combination. be able to. Among these, carbon black is preferably used, and ketjen black and acetylene black are more preferable.

The particle size of the carbon material used as the conductive aid is, for example, an average particle size measured by dispersing these fine particles in a medium using the laser scattering particle size distribution meter (for example, “LA-920” manufactured by HORIBA, Ltd.). The diameter (D _50% ) is preferably 0.01 μm or more, more preferably 0.02 μm or more, and preferably 10 μm or less, more preferably 5 μm or less.

<Negative electrode mixture layer, current collector, etc.>
In the negative electrode mixture layer, it is preferable that the total amount of the negative electrode active material is 80 to 99% by mass and the amount of the binder is 1 to 20% by mass. In addition, when a conductive material is separately used as the conductive auxiliary agent, these conductive materials in the negative electrode mixture layer are used in a range in which the total amount of the negative electrode active material and the binder amount satisfy the above-described preferable values. It is preferable. The thickness of the negative electrode mixture layer is preferably 10 to 100 μm per one surface, for example.

  As the current collector for the negative electrode, a foil made of copper or nickel, a punching metal, a net, an expanded metal, or the like can be used, but a copper foil is usually used. In the negative electrode current collector, when the thickness of the entire negative electrode is reduced in order to obtain a battery having a high energy density, the upper limit of the thickness is preferably 30 μm, and the lower limit is 5 μm in order to ensure mechanical strength. Is desirable.

<Separator>
The separator may be a polyolefin porous film made of, for example, polyethylene or polypropylene, which is used for a normal lithium ion secondary battery.

  Especially when the polyethylene porous membrane is used as the separator, since the melting point of polyethylene is about 130 ° C., when the inside of the battery exceeds 130 ° C., the separator melts and contracts, and the positive electrode and the negative electrode are short-circuited. Sometimes. Therefore, in order to improve safety at high temperatures, for example, a separator in which a heat resistant resin or a heat resistant inorganic filler is laminated may be used.

  In particular, when a laminated separator having a porous layer (I) mainly composed of a thermoplastic resin and a porous layer (II) mainly comprising a filler having a heat resistant temperature of 150 ° C. or higher is used, the separator is It is desirable because it combines high shutdown characteristics and heat resistance (heat shrinkage resistance).

  In this specification, “heat-resistant temperature is 150 ° C. or higher” means that deformation such as softening is not observed at least at 150 ° C.

  The porous layer (I) according to the separator is mainly for ensuring a shutdown function, and when the battery reaches or exceeds the melting point of the thermoplastic resin that is the main component of the porous layer (I), The thermoplastic resin related to the porous layer (I) melts and closes the pores of the separator, thereby causing a shutdown that suppresses the progress of the electrochemical reaction.

  The thermoplastic resin that is the main component of the porous layer (I) is a resin having a melting point, that is, a melting temperature of 140 ° C. or less measured using a differential scanning calorimeter (DSC) in accordance with JIS K 7121. Specific examples include polyethylene. In addition, as the form of the porous layer (I), a microporous film usually used as a battery separator or a base material such as a nonwoven fabric is coated with a dispersion containing polyethylene particles and dried. Examples thereof include sheet-like materials such as those obtained. Here, among the total volume of the constituent components of the porous layer (I) [the total volume excluding the pores. The same applies to the volume content of the constituent components of the porous layer (I) and the porous layer (II) relating to the separator. ], The volume content of the main thermoplastic resin is 50% by volume or more, and more preferably 70% by volume or more. For example, when the porous layer (I) is formed of the PE microporous film, the volume content of the thermoplastic resin is 100% by volume.

  The porous layer (II) according to the separator has a function of preventing a short circuit due to direct contact between the positive electrode and the negative electrode even when the internal temperature of the battery rises, and has a heat resistance temperature of 150 ° C. or higher. The function is secured by. In other words, when the battery becomes hot, even if the porous layer (I) shrinks, the porous layer (II) which does not easily shrink can cause the positive and negative electrodes directly when the separator is thermally shrunk. It is possible to prevent a short circuit due to the contact of. Moreover, since this heat-resistant porous layer (II) acts as a skeleton of the separator, the thermal contraction of the porous layer (I), that is, the thermal contraction of the entire separator itself can be suppressed.

  The filler related to the porous layer (II) has a heat-resistant temperature of 150 ° C. or higher, is stable with respect to the electrolyte solution of the battery, and is electrochemically stable that is not easily oxidized or reduced in the battery operating voltage range. If present, inorganic particles or organic particles may be used, but fine particles are preferable from the viewpoint of dispersion and the like, and inorganic oxide particles, more specifically, alumina, silica, and boehmite are preferable. Alumina, silica, and boehmite have high oxidation resistance, and the particle size and shape can be adjusted to the desired numerical values, making it easy to accurately control the porosity of the porous layer (II). It becomes. In addition, the filler whose heat-resistant temperature is 150 degreeC or more may use the thing of the said illustration individually by 1 type, and may use 2 or more types together, for example.

<Electrolyte>
As the non-aqueous electrolyte solution according to the battery of the present invention, a solution in which a lithium salt is dissolved in a non-aqueous solvent is usually used.

  Examples of the solvent for the non-aqueous electrolyte include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), γ -Butyrolactone (γ-BL), 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), 2-methyltetrahydrofuran, dimethyl sulfoxide (DMSO), 1,3-dioxolane, formamide, dimethylformamide (DMF), Dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivative, sulfolane, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative Aprotic organic solvents such as conductors, diethyl ether, and 1,3-propane sultone can be used alone or as a mixed solvent in which two or more are mixed.

The inorganic ion salt according to the non-aqueous electrolyte solution, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) 2 , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group] At least one selected from the group consisting of: The concentration of these lithium salts in the non-aqueous electrolyte is preferably 0.6 to 1.8 mol / l, and more preferably 0.9 to 1.6 mol / l.

  Non-aqueous electrolytes include cyclohexylbenzene or its derivatives (1-fluoro-2-cyclohexylbenzene, 1-fluoro-3-cyclohexylbenzene, 1-fluoro-4-cyclohexylbenzene, 1-chloro-4-cyclohexylbenzene, 1 -Bromo-4-cyclohexylbenzene, 1-iodo-4-cyclohexylbenzene, etc.) are preferably used.

  Cyclohexylbenzene or a derivative thereof has an action of decomposing and generating gas when the battery is overcharged. Therefore, by using a nonaqueous electrolytic solution containing cyclohexylbenzene or a derivative thereof, the operability of the pressure-sensitive safety means during battery overcharge can be further enhanced.

  As for cyclohexylbenzene or a derivative thereof, those exemplified above may be used alone, or two or more thereof may be used in combination.

  Content of cyclohexylbenzene or derivative thereof related to non-aqueous electrolyte used in battery (if only one of these is used, it is the content, and when using two or more in combination, the total of them) The same applies to the content of cyclohexylbenzene or a derivative thereof relating to the non-aqueous electrolyte used in the battery.) From the viewpoint of ensuring the above-mentioned effects due to their use, the content of the non-aqueous electrolyte is as follows. The total amount of the solvent is preferably 1.0% by mass or more, and more preferably 1.5% by mass or more.

  However, cyclohexylbenzene or a derivative thereof in the non-aqueous electrolyte may be gradually decomposed even during normal use of the battery, causing an increase in resistance of the positive electrode and clogging of the separator. Therefore, from the viewpoint of avoiding the above problem due to the use of cyclohexylbenzene or a derivative thereof, the content of cyclohexylbenzene or a derivative thereof in the nonaqueous electrolytic solution used in the battery is 3.0% in the total amount of the solvent of the nonaqueous electrolytic solution. The content is preferably at most mass%, more preferably at most 2.0 mass%.

  In addition, the non-aqueous electrolyte used in the battery has an acid anhydride, a sulfonic acid ester, a dinitrile, for the purpose of further improving the safety such as further improvement of charge / discharge cycle characteristics and high-temperature storage and prevention of overcharge. 1,3-propane sultone, diphenyl disulfide, biphenyl, fluorobenzene, t-butylbenzene, vinylene carbonate and its derivatives, halogen-substituted cyclic carbonates (such as 4-fluoro-1,3-dioxolan-2-one), etc. These additives (including these derivatives) can also be added as appropriate.

  Further, a gelling agent made of a polymer or the like may be added to the nonaqueous electrolytic solution and used as a gel (gel electrolyte).

<Battery exterior>
The battery exterior body only needs to have pressure-sensitive safety means, for example, steel or aluminum (aluminum alloy) tubular cans (cylindrical or square tubular) cans, and metal is deposited. A laminate outer package made of a laminate film can be used.

  FIG. 1 is a longitudinal sectional view showing an example of a lithium ion secondary battery according to the present invention. The lithium ion secondary battery shown in FIG. 1 is an example of a cylindrical battery provided with explosion-proof means including a current interruption mechanism as a pressure-sensitive safety means.

  In the lithium ion secondary battery shown in FIG. 1, the positive electrode 1 and the negative electrode 2 are spirally wound via a separator 3 and are housed in a battery case (exterior body) 5 together with a nonaqueous electrolyte solution 4 as a wound electrode body. Has been. In FIG. 1, in order to avoid complication, a current collector used for manufacturing the positive electrode 1 and the negative electrode 2 is not shown.

  An insulator 6 (for example, PP) is disposed on the bottom of the battery case 5 (for example, made of stainless steel) prior to the insertion of the wound electrode body. The sealing plate 7 (for example, made of aluminum) has a disk shape, a thin portion 7a is provided at the center thereof, and a pressure introduction port for allowing the battery internal pressure to act on the explosion-proof valve 9 around the thin portion 7a. A hole 7b is provided. And the protrusion part 9a of the explosion-proof valve 9 is welded to the upper surface of this thin part 7a, and the welding part 11 is comprised. The thin-walled portion 7a provided on the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 are shown only on the cut surface for easy understanding on the drawing, and the contour behind the cut surface is The illustration is omitted. In addition, the welded portion 11 of the thin-walled portion 7a of the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 is also illustrated in an exaggerated state so as to facilitate understanding on the drawing.

  The terminal plate 8 (for example, made of rolled steel with a nickel plating on the surface) has a hat shape with a peripheral edge portion, and the terminal plate 8 is provided with a gas discharge port 8a. The explosion-proof valve 9 (for example, made of aluminum) has a disk shape, and has a projecting portion 9a having a tip portion on the power generation element side (lower side in FIG. 1) at the center thereof, and a thin-walled portion 9b. As described above, the lower surface of the protruding portion 9a is welded to the upper surface of the thin portion 7a of the sealing plate 7 to constitute the welded portion 11. The insulating packing 10 (for example, made of PP) has an annular shape and is disposed at the upper portion of the peripheral edge of the sealing plate 7, and the explosion-proof valve 9 is disposed on the upper portion thereof, and insulates the sealing plate 7 and the explosion-proof valve 9. . The annular gasket 12 is made of, for example, PP, the lead body 13 (for example, made of aluminum) connects the sealing plate 7 and the positive electrode 1, the insulator 14 is disposed on the upper part of the wound electrode body, the negative electrode 2 and the battery case. 5 is connected with a lead body 15 (for example, made of nickel).

  In this battery, the thin-walled portion 7a of the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 are in contact with each other at the welded portion 11, the peripheral portion of the explosion-proof valve 9 and the peripheral portion of the terminal plate 8 are in contact with each other. Since the positive electrode 1 and the terminal plate 8 are connected to the sealing plate 7 by the positive lead 13, the positive electrode 1 and the terminal plate 8 are electrically connected by the lead 13, the sealing plate 7, the explosion-proof valve 9, and their welded portions 11. Connection is obtained and functions normally as an electrical circuit.

  If an abnormal situation occurs in the battery, such as when the battery is overcharged or the battery is exposed to high temperatures, and the internal pressure of the battery rises due to the generation of gas inside the battery, the internal pressure The rise is designed so that the explosion-proof means operates as follows to prevent the battery from bursting. First, the current interruption mechanism is such that the central portion of the explosion-proof valve 9 is deformed in the internal pressure direction (upward direction in FIG. 1), and accordingly, a shearing force is applied to the thin portion 7a integrated in the welded portion 11. When the thin-walled portion 7a is broken or the welded portion 11 between the projecting portion 9a of the explosion-proof valve 9 and the thin-walled portion 7a of the sealing plate 7 is peeled off, the current is cut off, and then the explosion-proof valve 9 When the thin-walled portion 9b provided on the base plate is cleaved, the gas is discharged from the gas discharge port 8a of the terminal plate 8 to the outside of the battery.

  The lithium ion secondary battery of this invention can be used for the same use as the conventionally known lithium ion secondary battery.

Example 1
<Positive electrode active material>
Lithium-containing composite oxide containing lithium and nickel represented by the composition formula shown in Table 1, nickel molar ratio ai (provided that i = 1 to 3) as shown in Table 2, average particle diameter Di (provided that I = 1 to 3) and the ratio of the average particle diameter Dave of all the positive electrode active materials to the average particle diameter Di of each positive electrode active material (= Di / Dave) is as shown in Table 3. Each positive electrode active material was prepared such that the total Nt of nickel mole content ratios was as shown in Table 2 at a mass ratio mi shown in Table 2 (where i = 1 to 3).

<Preparation of positive electrode>
Prepared as described above, 100 parts by mass of the total positive electrode active material, 20 parts by mass of a solution of N-methyl-2-pyrrolidone (NMP) containing 10% by mass of PVdF as a binder, and artificial graphite as a conductive assistant 1 part by mass and 1 part by mass of ketjen black were kneaded using a biaxial kneader, and NMP was added to adjust the viscosity to prepare a positive electrode mixture-containing paste. The positive electrode mixture-containing paste is intermittently applied to both sides of an aluminum foil (positive electrode current collector) having a thickness of 15 μm while adjusting the thickness, dried, and then subjected to a calendering process so that the total thickness becomes 130 μm. The thickness of the positive electrode mixture layer was adjusted, and the positive electrode was produced by cutting the positive electrode mixture layer to have a width of 55 mm and a length of 886 mm.

<Production of negative electrode>
A composite in which the surface of SiO is coated with a carbon material as the negative electrode active material (the average particle diameter is 5 μm, and the amount of the carbon material in the composite is 10% by mass; hereinafter referred to as “SiO / carbon material composite”) and Graphite having an average particle size of 16 μm mixed in such an amount that the amount of SiO / carbon composite is the mass% shown in Table 3: 97.5 mass%, SBR: 1.5 mass%, and carboxymethyl cellulose (Thickener): 1% by mass was mixed with water to prepare a slurry for forming a negative electrode mixture layer. This slurry for forming the negative electrode mixture layer is applied to both sides of a copper foil (thickness: 8 μm) as a current collector, vacuum-dried at 120 ° C. for 12 hours, and further subjected to a press treatment to a total thickness of 110 μm. Then, the thickness of the negative electrode mixture layer was adjusted so that the negative electrode was prepared by cutting to a width of 57 mm and a length of 1025 mm.

<Preparation of separator>
Modified polybutyl acrylate resin binder: 3 parts by mass, boehmite powder (average particle size: 1 μm): 97 parts by mass, and water: 100 parts by mass were mixed to prepare an insulating layer forming slurry. This slurry was applied to one side of a polyethylene microporous membrane (thickness 14 μm) for a lithium ion battery (coating thickness 2 μm) and dried to obtain a separator.

<Production of electrode body>
A positive electrode lead was welded to the positive electrode (part where the mixture layer was not applied), and a negative electrode lead was welded to the negative electrode. The positive electrode and the negative electrode welded with the leads were superposed through the separator and wound in a spiral shape to produce a wound electrode body.

<Preparation of non-aqueous electrolyte>
LiPF ₆ was dissolved in a mixed solvent of dimethyl carbonate and ethylene carbonate (volume ratio 2: 1) at a concentration of 1.4 mol / l, and 3% by mass of vinylene carbonate (VC) and cyclohexylbenzene (CB) 2. 0% by mass was added to prepare a non-aqueous electrolyte.

<Battery assembly>
The wound electrode body was inserted into the cylindrical battery case 5, and the negative electrode lead body 15 was welded to the bottom of the battery case 5. The positive electrode lead body 7 was welded to the sealing body 7 integrated with the terminal plate 8, the explosion-proof valve 9, the insulating packing 10 and the gasket 12. Thereafter, the non-aqueous electrolyte is poured into the battery case 5, and the terminal plate 8 is crimped and sealed with the battery case 5 via the gasket 12, and the configuration shown in FIG. 1 has an outer diameter of 18 mm and a height of 65 mm. A cylindrical battery was produced.

<Battery capacity measurement>
After carrying out constant current-constant voltage charging (total charging time: 2.5 hours) with a constant current of 0.75 A and a constant voltage of 4.2 V, constant current discharging (discharging end voltage: 2. 5V) and the initial discharge capacity was measured. The results are shown in Table 4.

<Cycle test>
After performing constant current-constant voltage charging (cut current is 0.1 A) with a constant current of 3.5 A and a constant voltage of 4.2 V, a constant current discharge (discharge end voltage: 2.5 V) was performed at 20 A. . With this as one cycle, 500 cycles of charge and discharge were repeated. The value obtained by dividing the discharge capacity at the 500th cycle by the discharge capacity at the first cycle is taken as the capacity retention rate (%), and the results are shown in Table 4.

<Overcharge test>
The battery was charged with a constant current of 1.5 A until it reached 15 V, and the presence or absence of the vent operation of the battery was examined. The results are shown in Table 4.

<External short circuit test>
After performing constant current-constant voltage charging (total charging time: 2.5 hours) with a constant current of 0.75 A and a voltage of 4.2 V, a resistance load of 9.8 mΩ is applied between the positive and negative terminals. And connected to the external circuit. The battery after the short circuit was observed to check for leakage from the battery.

(Examples 2-18, Comparative Examples 1-8)
In the same manner as in Example 1, a lithium-containing composite oxide containing lithium and nickel represented by the composition formula shown in Table 1, nickel molar ratio ai (where i = 1 to 3), average particle diameter Di ( However, the ratio of the average particle diameter Dave of all the positive electrode active materials to i = 1 to 3) and the average particle diameter Di of each positive electrode active material (= Di / Dave) is as shown in Tables 2 and 3. Prepare all positive electrode active materials by using the materials 1 to 3 and the mass ratio mi (where i = 1 to 3) shown in Table 2 so that the total Nt of nickel mole content ratios is as shown in Table 2. did. Moreover, what adjusted the quantity of SiO / carbon material composite to become the mass% shown in Table 3 was made into the negative electrode active material. Except for these, a cylindrical battery having the structure shown in FIG. 1 was produced in the same manner as in Example 1, and the battery capacity measurement, cycle test, overcharge test, and external short circuit test were performed in the same manner as in Example 1. did. The results are shown in Table 4.

  Comparative Examples 1, 4, 8 and 9 in which the total Nt of nickel mole content ratios is 0.6 or less are inferior in capacity and cycle characteristics and inferior in vent operability as compared with the batteries of the examples. It was. In Comparative Examples 2, 5, 6, and 7 where Nt is 0.75 or more, although the capacity and cycle characteristics have reached a certain level, the leakage of the electrolyte solution was found in the external short circuit test, and the safety Left a challenge. Further, Comparative Example 3 using only one kind of positive electrode active material (positive electrode active material 2) also had good battery capacity and cycle characteristics, but liquid leakage was found in the external short circuit test.

  INDUSTRIAL APPLICABILITY The present invention can be used for a lithium ion secondary battery including a battery case in which an electrode body and the like are accommodated.

1 Positive electrode 2 Negative electrode 3 Separator

Claims (3)

In a non-aqueous electrolyte secondary battery loaded with at least a non-aqueous electrolyte, an electrode body configured by winding a negative electrode, a positive electrode, and a separator in a bottomed can,
The negative electrode includes a material containing Si and O as constituent elements (provided that the atomic ratio x of O to Si is 0.5 ≦ x ≦ 1.5) as a negative electrode active material,
The positive electrode includes a lithium-containing composite oxide containing lithium and nickel as a positive electrode active material,
A positive electrode active material 1 in which the molar ratio a1 of nickel in the lithium-containing composite oxide is 0.30 to 0.59;
A positive electrode active material 2 in which the molar ratio a2 of nickel in the lithium-containing composite oxide is 0.60 to 0.79;
The positive electrode active material 3 in which the molar ratio a3 of nickel in the lithium-containing composite oxide is 0.80 to 0.95, and the total Nt of nickel molar ratios represented by Formula 1 is 0.6 <Nt <Lithium ion secondary battery characterized by satisfying <0.75.
(Formula 1)

ai: nickel mole fraction contained in each positive electrode active material of positive electrode active materials 1 to 3 mi: content mass ratio of each positive electrode active material of positive electrode active materials 1 to 3 contained in all positive electrode active materials   2. The ratio of the average particle diameter Dave of all the positive electrode active materials and the average particle diameter Di of the positive electrode active materials 1 to 3, both of Di / Dave is 0.7 to 1.3. Lithium ion secondary battery.   3. The lithium ion secondary battery according to claim 1, wherein each of the positive electrode active material 1, the positive electrode active material 2, and the positive electrode active material 3 has an average particle diameter of 4 to 15 μm.

Images